Generally, there have been known a force curve measuring technique that uses an atomic force microscope (AFM: Atomic Force Microscope) and a measuring technique using a surface-force measuring device (see, for example, a patent document 1). When a probe, which is attached to a tip end of a cantilever (or cantilever spring), approaches an object to be measured (or a specimen), the probe is attracted to the object by a surface force acting between the object and the probe, and the probe is then brought into contact with the object. When the probe is forced in a direction away from the object, the probe continues to stay in contact with the object due to the surface force until the probe is separated from the object at a certain position. The surface force is calculated by multiplying an amount of flexure of the cantilever (i.e., a displacement of the probe) by a spring constant of the cantilever.
When the surface force is measured using the cantilever, a measurable range for the surface force depends on the spring constant of the cantilever. If the spring constant is too small relative to the surface force to be measured, the cantilever is bent to a large extent. When the cantilever is bent, the probe is displaced in a circular path around a fulcrum at which the cantilever is fixed. Therefore, a direction in which the probe is displaced does not align with a direction in which the surface force acts. As a result, an error between a measured value and an actual surface force may arise due to the fact that the direction in which the probe is displaced does not align with the direction in which the surface force acts, and such an error cannot be ignored. On the other hand, if the spring constant is too large relative to the surface force to be measured, a measurement error of the displacement of the probe may arise because the amount of flexure of the cantilever is small.
For the above reasons, in order to accurately measure the surface force using a cantilever, it is necessary to choose a cantilever having a spring constant suitable for a measurement range for the surface force. However, it is time-consuming to replace a cantilever with another for every measurement condition. Such disadvantages have been neither recognized nor suggested as problems.